The invention relates to a method for preparing acrylic polyols. More particularly, the invention relates to a method for preparing an acrylic polyol that causes low color increase when mixed with UV light stabilizers.
High-solids acrylic polyols are of low molecular weights. Typically, high-solids acrylic polyols have a number average molecular weight (Mn) from 1,000 to 5,000. Many problems are associated with the molecular weight reduction of acrylic polyols.
First, the molecular weight reduction is often compensated by an increase in hydroxyl number. Typically, high-solids acrylic polyols have a hydroxyl number in the range of 60 mg KOH/g to 160 mg KOH/g. The lower the molecular weight, the higher the hydroxyl number is needed. The increased hydroxyl number requires an increase in the amount of crosslinking agents in the subsequent coating formulations. Crosslinking agents such as isocyanates are expensive, and thus the coating cost is increased.
Second, low molecular weight resins have reduced weatherability because the increased concentration of the polymer chain ends. The polymer chain ends are often the starting points for photodegradation. Thus, the coatings formulated from high-solids acrylic polyols require an increase in the amount of ultraviolet (UV) light stabilizers. The latter are very expensive.
Finally, making low molecular weight acrylic polyols requires a large amount of free radical initiator. Typically, 2 wt % to 10 wt % of initiator based on the amount of monomers is used for making high-solids acrylic polyols. The initiators are expensive.
Furthermore, we have found that the acrylic polyols made with high initiator concentrations cause significant color increase when mixed with UV light stabilizers. In summary, new methods are needed for preparing high-solids acrylic polyols. Ideally, the method would use a low initiator concentration and produce acrylic polyols having low color increase when mixed with UV light stabilizers.
The invention is a method for preparing low-yellowing acrylic polyols. The method comprises free radically copolymerizing an allylic alcohol, an alkyl acrylate or methacrylate, and optionally a vinyl comonomer selected from the group consisting of vinyl aromatics, vinyl ethers, and vinyl esters. The polymerization is performed at an initiator concentration less than or equal to 0.8 wt % of the total amount of monomers. Surprisingly, the resulting acrylic polyol made at such a low initiator concentration remains at very low molecular weights and is suitable for high-solids coating applications.
More importantly, we found that an acrylic polyol made with a higher initiator concentration gives a significant color increase (greater than 100%) when the acrylic polyol is mixed with UV light stabilizers. In contrast, the resulting acrylic polyol of the invention has significantly lower color increase when mixed with UV light stabilizers.